To review, Thomas’s position is that it is morally permissible to kill an assailant in self-defense, even when the assailed is not a public official or soldier, but only when it is necessary to preserve one’s life and only because the action that brings about the death of the assailant is a foreseen but unintended consequence of the action. In order to be morally licit, the lethal action taken in self-defense must not spring from an intention to kill the assailant; it can only spring from the intention to preserve one’s own life.
The first two objections Thomas considers attempt to put him at odds with Augustine. For Augustine says (in a letter) that “I do not agree with the opinion that one may kill a man lest one be killed by him.” And elsewhere (On Free Choice of the Will) an argument can be cobbled together for the same conclusion. Thus it appears that according to Augustine, Thomas is wrong.
Thomas’s reply is that in the letter in question (scroll down to section 5) Augustine does not contradict him because Augustine means only that “one may [not intend to] kill a man lest one be killed by him.” And this agrees with what Thomas has said: In defending my own life, I cannot intend to kill the assailant.
On the face of it, Thomas’s interpretation isn’t obvious to me. It’s true that a little later in the letter, Augustine says, “The precept, ‘Resist not evil,’ was given to prevent us from taking pleasure in revenge, in which the mind is gratified by the sufferings of others, but not to make us neglect the duty of restraining men from sin.” (This line of reasoning also summarizes Thomas’s reply to the fifth objection about a passage from Romans 12:19, which seems to prohibit self-defense.) The point Augustine makes suggests that the command to “resist not evil” is not given to prevent a man from acting with the intention of preserving his own life, but it doesn’t make that clear.
The second objection refers us to Augustine’s On Free Choice of the Will. (The relevant section is 1.5, which is one source of the famous phrase “an unjust law is no law at all.”) Here Thomas’s interpretation is that what Augustine really means (again) is that it is ultimately unlawful to defend one’s life (with lethal force) for the wrong reason, namely, with the intention of killing the assailant. On this point all I can say (again) is that I have read the relevant section of On Free Choice of the Will a number of times, and it isn’t obvious to me that Thomas’s interpretation is correct. Perhaps someone with more knowledge of this text can defend Thomas. I cannot. (In the first place, which one of the characters is presenting Augustine’s position? “Augustine” or “Evodius” or both? In the second place, the position that seems to stand is that the positive (human) law permits lethal self-defense even though those acting so in self-defense will be subject to a divine law.)
The third objection (which raises a question I was curious about before reading this passage) has to do with whether a cleric is permitted to remain a cleric after killing another person in self-defense. Thomas’s answer is no. This is because the question of “irregularity” (not adhering to the rule (regula) of Holy Orders) is not only a matter of intention but also of act. (I think the same distinction is present in Old Testament law in the distinction between “being guilty” and “being unclean.” Both are bad, but only the first involves intention.) Note that Thomas does not say whether the cleric's irregular status is partial or total, perpetual or temporal. These further clarifications probably depend on circumstances.
The fourth objection is useful for clarifying Thomas’s point about the double effect of the act of self-defense. The objection is that since no one is morally permitted to fornicate in order to save his or her life, and since murder is a more grievous sin than fornication, so no one is morally permitted to do commit a more grievous sin than fornication, namely, murder, in order to preserve one’s life.
Thomas’s reply is that “The act of fornication or adultery is not necessarily directed to the preservation of one’s own life, as is the act whence sometimes results the taking of a man’s life.” This point is clear enough, I think. The idea is that the act of fornication is not “necessarily directed” to saving one’s life. Suppose a male assailant promises not to harm his female victim if she consents to have sex with him (putting aside questions of the legitimacy of such “consent,” which surely is not consent in this case.) Thomas’s point is that it wouldn’t be morally permissible for the victim to consent to this because the act of sex is not necessarily directed to letting her go. That is, whether her life is preserved depends ultimately not upon whether she consents to have sex but on whether the assailant chooses to let her go. This is unlike an act of self-defense (say, punching or shooting) which does not depend on a further action (choice) in order to secure one’s own life. The act of shooting that is done with the intention of preserving one’s life is the very same act that causes the assailant to die. So Thomas’s point is that the difference in the grievousness between murder and fornication is not relevant here. What is relevant is the connection between an act and its effect(s).